Hydrostatically driven tractors used primarily for swathing are commonly used and readily available. The tractor carries at a forward end on suitable sprung supports a header for cutting standing crop with the crop being transported on the header to a suitable discharge location generally centrally of the tractor performing a swath in the field from the cut crop.
Such tractors generally include a pair of front wheels just behind the header which are fixed to the frame of the tractor so that they remain at an angle parallel to each other and parallel to a center line of the tractor. The tractor is supported at the rear end by a pair of castor wheels which are attached to the frame by a conventional castor system at the ends of a rear axle so that the wheels trail behind a vertical pivot mount for the wheels and are free to swivel through 360° around the pivot amount.
The front wheels only are driven and are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castoring action.
It is also known that such tractors can travel more effectively at high speed when reversed in direction so that the driven wheels are at the rear and the castor wheels at the front. The castor wheels of course rotate through 180° to trail behind the vertical pivot which is now forward of the castor wheels as the tractor moves at relatively high speed in this reversed direction.
Castor shimmy is an inherent problem on tractors of the above type. Recent machines have been increased in speed up to a maximum road speed of 24 mph instead of 16 which is the standard in the industry now. Speed aggravates the shimmy problem.
Loss of ground contact (going over bumps) also aggravates the problem and as a result weight is often added to prevent this. This results in increased power requirements and increased fuel consumption.
Friction devices on the swivel of the castor are often used as a common means to prevent shimmy, but they have their limitations as discussed hereinafter.